430 PHYSIOLOGY CHAP. 



the same experiments with more discretion. Poiseuille (1831) 

 applied his haemodynamometer to the veins, and attempted to 

 reduce to figures the force of the aspiration exerted in the 

 inspiratory act, and to establish at what distance from the thorax 

 its influence ceases. With the discovery of Ludwig's Kymograph 

 (1847) began the series of researches by the graphic method, which 

 were directed to the more exact determination of the influence 

 of respiratory mechanics upon blood pressure in the veins and 

 arteries (see p. 242). 



In Fig. 194 we have a very clear representation of the intra- 

 thoracic respiratory curves and the simultaneous curves of blood 

 pressure in the vena cava superior. Apart from the secondary 

 oscillations which depend on cardiac rhythm (see Chap. VII. 10), 

 it will be seen that the respiratory curves follow the same course, 

 and coincide with them, apart from a slight delay which the curve 

 of venous pressure exhibits in relation to that of intrathoracic 



FIG. 194. Respiratory oscillations of intrathoracic pressure (T) and pressure in vena cava superior 

 (C) in anaesthetised and tracheotomised dog. (Luciani.) T, taken with oesophageal explorer ; 

 C, with water manometer both connected, to Marey's recording tambours. 



pressure, which is probably dependent upon the presence of the 

 water manometer. It may therefore be concluded that the negative 

 pressure of the intrathoracic wave suffers diminution during the 

 inspiratory act, so that the velocity with which the blood flows 

 from the extrathoracic into the intrathoracic vein and the heart, 

 increases proportionately. The opposite occurs during the ex- 

 piratory act. 



When the effects of the respiratory movements are exaggerated 

 by constriction of the lumen of the tracheal tube, the respiratory 

 curves become more extensive, whether they are transmitted 

 from the oesophageal sound or from the vena cava superior (Fig. 

 195). 



The impulse given to the venous circulation by the inspiratory 

 movement is not counterbalanced by the opposite effect of the 

 expiratory movement. Expirations are, in fact, always somewhat 

 slower than inspirations; further, intrathoracic pressure always 

 remains negative even during the ordinary expiratory acts, so that 



